Oversampling for Nonlinear Waveshaping: Choosing the Right Filters
Julen Kahles, Fabián Esqueda, Vesa Välimäki · Journal of the Audio Engineering Society · 2019
An assessment of filters for classic oversampled audio waveshaping schemes is carried out in this paper, pursuing aliasing reduction.For this purpose, the quality measure of the Aweighted noise-to-mask ratio is computed for test tones covering the frequency range from 27.5 Hz to 4.186 kHz, sampled at 44.1 kHz, and processed at eight-times oversampling.All filters are designed to have their passband contained within a ±1 dB range and to display a minimum stopband attenuation value of 40 dB.Waveshaping of sinusoids via hard clipping is investigated: the spectral enrichment due to the discontinuities introduced by its nonlinear transfer function maximizes aliasing distortion.The obtained results suggest that linear interpolation equalized with a high shelving filter is a sufficiently good method for upsampling.Concerning decimation, the interpolated FIR, elliptic, and cascaded integrator-comb filters all improve the results with respect to the trivial case.Regarding performance, the cascaded integrator-comb filter is the only tested decimation filter that achieves perceptually sufficient aliasing suppression for the entire frequency range when combined with the linear interpolator.